It is known to fit the rear window opening of a truck cab with a sliding window assembly consisting of a pair of stationary window panes mounted upward in the window opening to define a gap therebetween, and one or two window panes retained along their upper and lower edges in guide tracks for horizontal sliding movement. In a window assembly having a single sliding pane, the pane is movable between a closed position wherein it completely closes the gap between the fixed panes and an open position wherein it is slid toward one side or the other to overlap one of the fixed panes and thereby leave the gap unobstructed. In a window assembly having two sliding panes, the panes are slidable between a closed position wherein the panes meet in the middle of the gap and an open position wherein they are slid in opposite directions away from one another, each overlapping one of the stationary panes.
The sliding panes may be moved between the open and closed positions by hand or by a power actuated window regulator. Various designs for such powered regulators have been proposed, most of them employing a reversible electric motor to drive a cable, slotted tape, or a rack-and-pinion mechanism to move the slidable pane or panes along the guide tracks between the open and closed position. These prior art regulators typically extend along the lower edge of the window opening adjacent to or below the lower guide track so that the regulator is relatively unobtrusive and blocks a minimum amount of the window opening. In such a case, the drive member of the regulator is attached to the window pane immediately adjacent its lower edge, either within or just above the lower guide track. This configuration results in the motive force supplied by the regulator being applied to the sliding pane adjacent its lower edge, well below the center of mass of the pane.
To ensure that the pane may slide freely and does not bind within the guide tracks, there must be some amount of vertical clearance between the upper edge of the sliding pane and the upper guide track when the lower edge of the window is resting upon the lower guide track. This clearance allows the sliding pane to tilt slightly within the guide tracks as the motive force urges the pane along its path of travel. The top edge of the pane tilts in the direction opposite the direction in which the force is applied.
If the sliding pane tilts only a small amount there is little adverse effect on operation of the window assembly except when the pane reaches the fully closed position. Since the pane tilts such that the upper edge of the pane lags the lower edge, a gap is created between the upper edge of the pane and the edge or grommet with which it seals. This gap is particularly pronounced in the case of a window assembly with two sliding panes, since both of the panes will tilt outwardly from one another adjacent their upper ends, thus creating a V-shaped gap between the panes.